Sur la tétrabromo-2, 2, 6, 6 méthyl-4 cyclohexanone: Mise en evidence d'un equilibre conformationnel par RMN à haut champ

High field n.m.r. spectra of 2,2,6,6-tetrabromo-4-methyl- and -4-tert-butylcyclohexanones ( 1 and 2 ) have been computed and simulated. From coupling constants (J_gem, J_ea, J_aa and ⁴J) and chemical shifts it is shown that 1 is in equilibrium in solution. This work supports the conclusions deduced from infrared data of 1 in the scissors vibration region δ(CH₃).     

Linear solvation energy relationships VIII—solvent effects on NMR spectral shifts and coupling constants

The solvatochromic comparison method is used to unravel solvent polarity and hydrogen bonding effects on a variety of NMR spectral shifts and coupling constants. Solvent effects are rationalized in terms of the solvatochromic parameters π*, δ, α and β. Properties analyzed include ¹⁹F shifts of 5-fluoroindole, ¹H shifts of fluorodinitromethane, tert-butanol, phenol, 2-methylbut-1-en-3-yne, and thioacetamide, ¹H and ¹³C shifts and J(¹³C¹H) coupling constants of chloroform, ¹³C shifts of acetone, ¹⁵N shifts of pyridine, ¹⁵N and ²⁹Si shifts of 1-methylsilatrane, and some J(¹¹⁹Sn,C,¹⁹F) coupling constants of polyalkyltin compounds.     

Group IV organometal derivatives of pyridine. I—A 1H and 13C NMR investigation of trialkylmetal derivatives of pyridine

The proton and carbon-13 NMR spectra of thirteen trialkylmetal derivatives of pyridine, several of which were previously unknown, have been recorded and analysed. The proton NMR spectra show variations in proton chemical shifts but not in proton-proton coupling constants when the metal substituent is changed; the ring proton-metal coupling constants ⁿJ(M? H) in the tin and lead derivatives correspond closely with the corresponding proton-proton couplings ⁿJ(H? H) in pyridine. The carbon-13 chemical shifts of the carbons bound to the metal can apparently be correlated with the electron-donating ability of the trialkylmetal group. In the trimethylstannylpyridines the value of ¹J(Sn? C_ring) varies greatly with the position of the Me₃Sn group.     

Carbon-13 nuclear magnetic resonance spectroscopy—VII:para-substituted fluorobenzenes

C-13 and F-19 NMR spectra of seventeen para-substituted fluorobenzenes were measured and the chemical shifts as well as coupling constants with respect to substituents were analysed. The chemical shifts of the fluorine, the C₁ and the C₂ atoms were found to depend on the total electron densities. In the case of the C₃ atom, the chemical shifts seem to depend on π-electron densities rather than the total electron densities. The present calculations also indicate that the chemical shift of the C₄ atom depends mainly on σ-electron densities due to the inductive effects of substituents. The strongest factor influencing the coupling constant, ⁿJ(C? F), is also considered to be the π-electron densities on the carbon atoms. In the case of the direct couplings, ¹J(C? F), the π-bond orders are important.     

15N NMR of 1,4‐dihydropyridine derivatives

In this article, we describe the characteristic ¹⁵N and ¹H_N NMR chemical shifts and ¹J(¹⁵N–¹H) coupling constants of various symmetrically and unsymmetrically substituted 1,4‐dihydropyridine derivatives. The NMR chemical shifts and coupling constants are discussed in terms of their relationship to structural features such as character and position of the substituent in heterocycle, N‐alkyl substitution, nitrogen lone pair delocalization within the conjugated system, and steric effects. Copyright © 2013 John Wiley & Sons, Ltd.     

31P-NMR-Daten heterocyclischer phosphine. Diastereomerenzuordnung an 1,3-Oxa-, 1,3-Aza- und 1,3-thiaphospholanen

The diastereomers of 16 1,3-oxa-, 1,3-aza- and 1,3- thiaphospholanes were assigned by means of the coupling constants ²J(P? C? H) and ³J(P? C? CH₃) and the linewidths of the ³¹P signals and ¹H chemical shifts of CH₃ groups. It is shown that the change in the ³¹P chemical shifts allows the estimation of the relative configuration in these compounds.     

Current developments in homonuclear and heteronuclear J‐resolved NMR experiments

Two‐dimensional J‐resolved (Jres) NMR experiments offer a simple, user‐friendly spectral representation where the information of coupling constants and chemical shifts are separated into two orthogonal frequency axis. Since its initial proposal 40 years ago, Jres has been the focus of considerable interest both in improving the basic pulse sequence and in its successful application to a wide range of studies. Here, the latest developments in the design of novel Jres pulse schemes are reviewed, mainly focusing on obtaining pure absorption lineshapes, minimizing strong coupling artifacts, and also optimizing sensitivity and experimental measurements. A discussion of several Jres versions for the accurate measurement of a different number of homonuclear (J_HH) and heteronuclear (J_CH) coupling constants is presented, accompanied by some illustrative examples.     

1H NMR parameters of the N-terminal 13-residue C-peptide of ribonuclease in aqueous solution

The ¹H NMR chemical shifts and the spin-spin coupling constants of the non-exchangeable protons of the N-terminal 13-residue C-peptide of ribonuclease A, obtained by cleavage of the enzyme with cyanogen bromide, have been measured in a 5 mM solution in D₂O (pH 3.0, 24°C) at 360 MHz. The titration parameters for end groups (Lys-1 and homo-Ser-13) and side chains (Lys-1, Glu-2, Lys-7, Glu-9 and His-12) have been determined. The chemical shifts, their temperature coefficients and the vicinal coupling constants, ³J(HNCH-α), for the exchangeable NH protons have been measured in a 5 mM solution in D₂O/H₂O (1:9 v/v) at pH 3.0. An assignment of observed signals to individual residue protons based on characteristic shifts, standard double resonance experiments, spectral simulations and titration shifts is proposed. All experimental evidence indicates that under the conditions studied the C-peptide is in a random coil form.     

NMR study of 2-alkoxy-1,3,2-dioxaphospholanes. II—molecular conformation of alkoxy substituents

The conformation of some 2-σ-1,3,2-dioxaphospholanes (σ=OAlkyl, CI) is studied. The determination of the ²J(POC) and ³J(POCC) coupling constants, which are influenced by the bulk of the alkoxy group, is a means of obtaining information about the rotation around the P? OR bond, which is dependent on steric interactions between the phosphorus lone pair, the alkoxy group and the substituents on the ring. When σ is a tert-butoxy group, a direct ‘through-space’ interaction is found between the phosphorus atom and one of the primary carbons of the OR group. If there is no substituent on the ring, the ³¹P chemical shifts are little affected by changes in OR; a diamagnetic effect is observed, however, in the case of the tert-butoxy group which is enhanced for the 4,4,5,5-tetramethyl derivatives.     

Indirect 13C?13C couplings in benzene and the contribution of their anisotropies to the direct couplings in the oriented molecule

The indirect ortho and para carbon-carbon coupling constants in benzene were determined utilizing the deuterium isotope effect on the carbon chemical shifts in 1-d₁- and 1,3,5-d₃-benzene. The measurements gave for J(CC, ortho) and J(CC, para) the values of 55.3 ± 0.5 Hz and 10.08 ± 0.10 Hz, respectively. The ratios of the direct dipolar C? C coupling constants were calculated using the previous results of 1-¹³C-benzene in an oriented phase. These ratios differ from the theoretical ones obtained assuming the benzene ring to be hexagonal, showing significant indirect contributions in the D values.     

NMR studies in the heterocyclic series. XVIII—Carbon-13 NMR study of azolium salts. Attempted correlation of chemical shifts with calculated charge densities

The carbon-13 chemical shifts and the ¹J(CH) coupling constants for 18 azolium salts (di-N-methyl-pyrazolium, -indazolium, -benzimidazolium and -benzotriazolium iodides) have been determined and assigned. The chemical shifts are discussed as a function of substituent effects, positive charge introduction and total electronic density (calculated by the CNDO/2 method). The general problem of correlations between chemical shifts and total charge densities in azoles is discussed. A statistical approach shows that these correlations are of poor quality.     

1H and 13C NMR spectral studies of some Mannich bases. Revised structures for the mono-substituted products from 3-hydroxy-2-pyridone

¹H and ¹³C chemical shifts and J_HH and J_CH coupling constants are reported for a series of mono-Mannich bases derived from 3-hydroxy-2-pyridone. These compounds should be regarded as the 4-isomers rather than the 6-isomers proposed previously.     

Carbon-13 nuclear magnetic resonance spectroscopy IX—monosubstituted ethylenes

Carbon-13 chemical shifts of sixteen monosubstituted ethylenes were obtained. In order to explain the chemical shifts, σ and π electron densities of these compounds are calculated by the σ-included ω-HMO method.

1 See Ref. 8.

A linear relationship exists between carbon-13 chemical shifts and the calculated electron densities, and also between substituent constants and electron densities. A slope of unity is obtained between the chemical shifts of α carbons of monosubstituted ethylenes and those of carbons adjacent to the substituents in monosubstituted benzenes. On the other hand, a plot of chemical shifts of C_ortho of benzene derivatives against that of the β carbon in ethylene derivatives gives a slope of 3. These slopes can be explained by the calculated electron densities. A slope of 4/3 is obtained between the direct coupling constant ¹J(C? H) of the α carbon in monosubstituted ethylenes and that in the corresponding substituted methanes.     

13C n.m.r. spectra of 2-substituted pyrimidines

¹³C n.m.r. chemical shifts and carbon-proton coupling constants of 2-substituted pyrimidines are reported. The carbon chemical shifts are correlated with π-electron densities. Substituents which cause deshielding at the directly bound carbon (e.g. NH₂, OCH₃ and F) exert a more powerful effect in the benzene series than in the pyridine or pyrimidine series. The carbon-proton coupling constants do not correlate with the electronegativity of the substituents. Carbon-proton coupling constants and proton-proton coupling constants over the same number of bonds do not obey the Karabatsos relationship. The changes in the carbon-proton coupling constants in 2(1H)-pyrimidinone and 2(1H)-pyrimidinethione which accompany anion and cation formation are reported.     

Studies in tertiary amine oxides part II—carbon-13 nuclear magnetic resonance spectra of selected acetylenic amines,their N-oxides and the Meisenheimer rearrangement products

Carbon-13 NMR chemical shifts and ¹J(CH), ²J(CH) and ³J(CH) coupling constants of selected saturated nitrogen heterocyclic molecules containing the acetylenic moiety have been determined. These NMR parameters have also been determined for the corresponding N-oxides and the Meisenheimer rearrangement products, the O-allenylhydroxylamines. The effect of the N-oxidation on the chemical shifts of the ring and the acetylenic carbon atoms is discussed.     

13C NMR spectra of non-labelled and 15N-mono-labelled azo dyes

¹³C NMR spectra of four types of azo coupling products from benzenediazonium chloride have been measured and interpreted, viz. hydrazo compounds with an intramolecular hydrogen bond (3-methyl-1-phenylpyrazole-4,5-dione 4-phenylhydrazone), azo compounds without an intramolecular hydrogen bond (4-hydroxyazobenzene), azo compounds with an intramolecular hydrogen bond (2-hydroxy-5-tert-butylazobenzene) and an equilibrium mixture of both the tautomers of 1-phenylazo-2-naphthol. The absolute values of the J(¹⁵N¹³C) coupling constants have been determined by recording the spectra of the ¹⁵N isotopomers, and have been used, in some cases, for ¹³C signal assignment. A relationship has been found between the chemical shifts of the C-1′ to C-4′ carbons of the phenyl group (from the benzenediazonium ion) or the ¹J(¹⁵N¹³C) coupling constant, and the composition of the tautomeric mixture.     

Multinuclear NMR Studies of the Substitlent Effects in N-Phenyl-P,P,P-Triarylphospha-λ5-Azenes,Triaryl-Phosphines and Triarylphosphine Oxides

Abstract

A series of N-phenyl-P,P,P-triarylphospha-λ⁵-azenes (1) as well as their ^l5N labeled analogs was synthesized. The ¹³C, ³¹P, and ¹⁵N NMR spectra of this series and those of two other series of related compounds, namely triarylphosphines (2) and triarylphosphine oxides (3), were measured and are reported. Many satisfactory correlations using the mono-substituent parameter (MSP) and the Taft dual-substituent parameter (DSP) treatments with the ¹³C substituent chemical shifts (SCS), ³¹P SCS, ¹⁵N SCS and the one bond P-N, P-C and C-N coupling constants were observed and will be discussed. Thus, for example, the ³¹P and ¹⁵N chemical shifts in 1 correlated with [sgrave]^?with negative slopes while the ³¹P chemical shifts in 3 correlated with those in 1 with a slope of 2.0. The ¹³C chemical shifts in 1 correlated excellently with the corresponding ones in 3 with slopes very close to unity. The substituent effects on the chemical shifts of the various nuclei were shown to be mainly due to changes in the charge distribution on those nuclei. In 1 the one bond P-N and P-C coupling constants correlated with [sgrave]_p and [sgrave]_R respectively. The one bond P-C coupling constants of 1 correlated quite well with those of 3 with a slope of 0.93 while the corresponding correlation of 1 with 2 was quite poor. Taft DSP treatment of ¹J_PCin 1 and 3 were quite similar, ρI and ρR were both negative and ρR was much larger than ρI. Series 2 showed behavior which was different from that shown by 1 and 3 but similar to that shown by other systems with a lone electron pair on the atom bound to the phenyl ring. The substituent effects on the one bond P-N, P-C and C-N coupling constants will be discussed in terms of bonding and hybridization changes between the directly bonded nuclei.     

Improved Purification of C60 and Formation of σ- and π-Homoaromatic methano-bridged fullerenes by reaction with alkyl diazoacetates

A rapid and inexpensive method for the large-scale purification of C₆₀ is the simple filtration of the toluenesoluble extract of commercial fullerene soot through a short plug of charcoal/silica gel with toluene as the eluent. Reactions of C₆₀ with ethyl and tert-butyl diazoacetates in refluxing toluene lead to the formation of the (alkoxycarbonyl)methylene-bridged isomers 1a – 3a and 1b – 3b , respectively, which can be equilibrated, upon further heating, into the single compounds 1a and 1b , respectively. Isomers 1a / b possess the methano bridge at the 6–6 ring junction, whereas structures 2a / b and 3a / b are bridged at the 6–5 junction. A dramatic influence of local and π-ring current anisotropic effects of the fullerene sphere on the NMR chemical shifts of the methine protons in the bridge is observed: the chemical shifts of the protons located over a pentagon ring in 2a / b and over a hexagon ring in 3a / b differ by Δδ = 3.47 and 3.45 ppm, respectively. The analysis of the ¹³C-NMR chemical shifts of the bridgehead C-atoms and the ¹J(C,H) coupling constants for the methano-bridge atoms reveals conclusively that the 6-5-ring-bridged structures 2a / 2b and 3a / 3b are π-homoaromatic (‘open’ transannular bond) and the 6-6-ring-bridged structures 1a / b are π-homoaromatic (‘closed’ transannular bond). The electronic absorption spectra show that π-homoconjugation in 2a / b and 3a / b represents a much smaller electronic perturbation of the original C₆₀ chromophore than σ-homoconjugation in 1a / b . The results of this study demonstrate an impressive linkage between the chemistry of methano-bridged annulenes and methano-bridged fullerenes.     

Sur la Similitude des Déformations du Noyau Cyclohexanonique Induites par les Groupes Tertiobutyle et gem-Diméthyle

X-ray and NMR (250 MHz) data for chlorinated 4,4-dimethylcyclohexanones lead to the following conclusions: carbonyl and chlorine substituent effects on ²J and ³J coupling constants are similar to those observed for 4-tert-butylcyclohexanones. In other respects, the gem dimethyl and the tert-butyl groups induce on the ring similar large ⁴J coupling constants (H-3′? C-3? C-4? C-5? H-5′); the results can be interpreted in terms of local gemoetric deformations and additivity of these deformations. The determination of dihedral angles by Lambert's method and from X-ray data shows the identity of the structures in the solid state and the dissolved state and confirms the great structural similarity between 4-tert-butyl- and 4,4-dimethylcyclohexanone derivatives.     

Characterization of tri-o-methylcellulose by one- and two-dimensional NMR methods

Tri-O-methylcellulose was prepared from partially O-methylated cellulose and its chemical shifts (¹H and ¹³C), and proton coupling constants were assigned using the following NMR methods: (1) One-dimensional ¹H and ¹³C spectra of the title compound were used to assign functional groups and to compare with literature data; (2) double quantum filtered proton–proton correlation spectroscopy (¹H, ¹H DQF-COSY) was used to assign the chemical shifts of the network of 7 protons in the anhydroglucose portion of the repeat unit; (3) the heteronuclear single-quantum coherence (HSQC) spectrum was used to establish connectivities between the bonded protons and carbons; (4) the heteronuclear multiple-bond correlation (HMBC) spectrum was used to connect the hydrogens of the methyl ethers to their respective sugar carbons; (5) the combination of HSQC and HMBC spectra was used to assign the ¹³C shifts of the methyl ethers; (6) all spectra were used in combination to verify the assigned chemical shifts; (7) first-order proton coupling constants data (J_H,H in Hz) were obtained from the resolution-enhanced proton spectra. The NMR spectra of tri-O-methylcellulose and other cellulose ethers do not resemble the spectra of similarly substituted cellobioses. Although the ¹H and ¹³C shifts and coupling constants of 2,3,6-tri-O-methylcellulose closely resemble those of methyl tetra-O-methyl-β-D -glucoside, there are differences with regard to the chemical shifts and the order of appearances of the resonating nuclei of the methyl ether appendages and the proton at position 4 in the pyranose ring. H4 in tri-O-methylcellulose is deshielded by the acetal system comprising the β-1→4 linkage, and it resonates downfield. H4 in the permethylated glucoside is not as deshielded by the equitorial O-methyl group at C4, and it resonates upfield. The order of appearance of the ¹H and ¹³C resonances in the spectra of the tri-O-methylcellulose repeat unit (from upfield to downfield) are H2 < H3 < H5 < H6a < H3a < H2a < pro R H6B < H4 < pro S H6A ≪ H1 and C6a < C3a < C2a < C6 < C5 < C4 < C2 < C3 ≪ C1, respectively. Close examination of the pyranose ring coupling constants of the repeat unit in tri-O-methylcellulose supports the ⁴C₁ arrangement of the glucopyranose ring. Examination of the proton coupling constants about the C5-C6 bond (J_5,6A and J_5,6B) in the nuclear Overhauser effect difference spectra revealed that the C6 O-methyl group is predominantly in the gauche gauche conformation about the C5-C6 bond for the polymer in solution. © 1999 John Wiley & Sons, Inc.* J Polym Sci A: Polym Chem 37: 4019–4032, 1999